Methods to increase crush resistance of silica sand proppant for fracking operations

ABSTRACT

Methods for cost effectively transforming lower quality silica sands into higher quality silica sands having increased crush resistance for use as proppants in fracturing operations involving subterranean hydrocarbon formations. One or more of an attrition process, a microwaving process and a tumbling process are disclosed and shown to be applied to silica sands having inferior physical properties in order to increase the performance of those silica sands as proppants in hydrocarbon formation fracturing operations.

RELATED APPLICATIONS

This application is a continuation, and claims priority to and thebenefit of U.S. Non-Provisional patent application Ser. No. 16/276,910,filed Feb. 15, 2019, titled “SYSTEMS AND METHODS TO STRENGTHEN SANDPROPPANT,” which is a divisional of U.S. Non-Provisional patentapplication Ser. No. 16/144,654, filed Sep. 27, 2018, titled “SYSTEMSAND METHODS TO STRENGTHEN SAND PROPPANT,” now U.S. Pat. No. 10,364,154,issued Jul. 30, 2019, which claims priority to and the benefit of U.S.Provisional Patent Application No. 62/690,073, filed Jun. 26, 2018,titled “SYSTEMS AND METHODS TO STRENGTHEN SAND PROPPANT,” the fulldisclosure of which is hereby incorporated herein by reference in itsentirety.

FIELD OF INVENTION

Embodiments relate to enhancing crush resistance and/or conductivity ofsilica sand used as proppant in fractured subterranean formations forhydrocarbon completions.

BACKGROUND

Frac sand is generally high-purity quartz sand that is added to frackingfluids used in subterranean hydrocarbon completions. The fracking fluidsalong with the frac sand are injected under pressure into oil and gaswells during the process of hydraulic fracturing or fracking. The highpressure of the fracking fluids on the subterranean hydrocarbonformation causes fractures of the hydrocarbon formation through whichthe fracking fluids travel carrying the frac sand. The frac sand settlesout of the fracking fluid within the fractures and prevents thefractures from closing when fracking fluid pressure dissipates. Thefractures, held open by the frac sand, provide hydrocarbons within thesubterranean formation a fluid pathway to the oil and/or gas well forproduction to the surface.

The oil and gas industry has placed heavy emphasis on best-in-class fracsand as being the primary proppant used in unconventional oil and gaswell completions. For example, billions of dollars of developmentcapital have been spent on mining and logistics infrastructure totransport superior sand from various regions of the United States to thehydrocarbon producing basins in the United States. The principle sourcesof preferred sands in the United States, for example, come from theupper Midwest of the United States, such as Wisconsin, and must beshipped by truck or rail to oil and gas plays in Texas, Oklahoma andNorth Dakota.

With the decrease in hydrocarbon prices in recent years, alternatives tothe transportation and logistics of moving Northern White or Ottawa sandfrom the upper Midwest have been considered to maintain oil and gasmargins. Resin coated sands and ceramics offer an alternative to rawquartz sand but at a higher cost than the transportation of NorthernWhite and Ottawa raw sands. Therefore, raw sands from the upper Midwestcontinue to make up the majority of the proppant market. Many companieshave recently invested in west Texas dune sand mines, for example, as alower cost alternative to such transportation. While less costlytransportation and logistics costs of west Texas sands provide strongeconomic incentives, Applicants recognized that the quality of suchsands is questionable.

Applicants also recognized, for example, that under harsh conditions,hydraulic fracturing propping agents, or proppants, such as frac sand,are subject to variables that diminish their effectiveness in ensuringhydrocarbon flow passages and hydrocarbon flow velocity. A desiredcharacteristic of the sand proppants is to be able to withstand highphysical pressures such that, when the fracking fluid pressure in thefractures is reduced, the resettling of the formation does not crush thefrac sand, thereby closing the fractures and, thus, the fluid pathwayfor the formation hydrocarbons to travel to surface via the well.

SUMMARY

Applicants recognized that superior results are achieved by deploying asand proppant that has the highest compressive strength and crushresistance while remaining economically feasible. Applicants alsorecognized that roundness of frac sand particles is one characteristicthat governs this degree of crushability. Frac sand particles that havegreater roundness tend to withstand greater physical pressures prior tobeing crushed, or being broken into fines. Applicants further recognizedthat another characteristic of frac sand that affects crush resistanceis the frac sand's content of high-strength alpha-quartz. Higheralpha-quartz content has been associated with greater degrees of crushresistance. Furthermore, Applicants recognized that sands having ahigher crush resistance tend to have a higher conductivity, orpermeability, which facilitates oil and gas production between the fracsand particles used as proppant in fractured hydrocarbon formations.

Thus, embodiments of systems and methods to enhance crush resistance andconductivity of lower quality sands found closer to hydrocarbonproducing formations and to compete economically with the transportationand logistics costs, for example, of upper Midwest frac sands, aredesired and herein provided. In one or more implementations, silica sandis strengthened and its crush resistance and conductivity enhancedthrough one or more processes of attrition, radiation exposure andtumbling. Thus, through one or more of these processes, a poor qualitysand that may not be ideal for use as frac sand may be structurallytransformed into a higher quality sand with individual sand grains thathave higher specific gravities and are more round and/or spherical,e.g., have less angularity.

In an attrition process, according to one or more embodiments, poorerquality sand may be washed with an acid solution to remove anynon-silicon dioxide impurities, thereby increasing the overallpercentage of silicon dioxide in the sand. In a radiation exposureprocess, according to one or more embodiments, the washed sand isexposed to microwave electromagnetic radiation to heat the individualsand particles. The addition of heat to the individual sand particlesmay weaken any bonding between the silicon dioxide molecules andimpurities while permitting the strengthening of silicon dioxidemolecule bonding. In a tumbling process, according to one or moreembodiments, the microwave-treated sand is tumbled soon after themicrowaving process and while such sand is still at a temperature abovethe ambient temperature. The individual sand grains, while at such anelevated temperature, may be easier to mold or shape into more round orspherical particles, having less angularity, through the physicalstriking of the individual sand grains against each other and the innerwalls of the tumbler. Thus, by one or more of these processes, theindividual sand grains may be made to have physical characteristics morelike those in higher quality frac sands having a higher alpha-quartzcrystallography.

An embodiment of a method of increasing the crush resistance of silicasand proppant used in fracturing operations involving one or moresubterranean hydrocarbon formations, for example, includes washingsilica sand proppant in a washer to remove impurities and exposing atleast a portion of the washed silica sand proppant to electromagneticradiation to impart heat thereto. The electromagnetic radiation, forexample, may be generated from a local electromagnetic radiation sourceand may have a frequency of between about 300 MHz and about 300 GHz. Anembodiment of a method of increasing crush resistance of silica sandproppant may also include tumbling the silica sand proppant in a sandtumbler within a preselected period of time after exposure to theelectromagnetic radiation.

An embodiment of a system to increase the crush resistance of silicasand proppant used in fracturing operations involving one or moresubterranean hydrocarbon formations, for example, includes a washerhaving a cavity to receive silica sand proppant and a wash solution, thewasher having an agitator to agitate the silica sand proppant and thewash water. The wash solution includes an acidic solution. An embodimentof the system also includes a microwave device having an interiorchamber positioned within an outer housing. The interior chamber ispositioned to receive and maintain the washed silica sand proppanttherein for a preselected microwave exposure time. The microwave devicealso may have a local radiation emitting source that emitselectromagnetic radiation into the interior chamber at a frequency ofbetween about 300 MHz and about 300 GHz. An embodiment of the systemalso may include a tumbler positioned to be rotated upon an axis thereofat a preselected rate of rotations per minute. The tumbler, for example,may have a barrel positioned to receive the silica sand proppant fromthe microwave device.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the disclosure willbecome better understood with regard to the following descriptions,claims, and accompanying drawings. It is to be noted, however, that thedrawings illustrate only several embodiments of the disclosure and,therefore, are not to be considered limiting of the disclosure's scope.

FIG. 1 is a schematic diagram of a sand sorting apparatus according toone or more embodiments of the disclosure having a trommel screen thatmay be used to segregate the various sizes of sand.

FIG. 2 is a schematic diagram of an apparatus that may be used to washraw silica sand in order to remove non-silicon dioxide impurities in anattrition process according to an embodiment of the disclosure.

FIG. 3 is a schematic diagram of a microwaving system that permitswashed silica sand to be exposed to microwave electromagnetic radiationon a continuous or semi-batch basis according to an embodiment of thedisclosure.

FIG. 4 is a schematic diagram of a microwaving system that may be usedto expose washed silica sand to microwave electromagnetic radiation on abatch basis according to an embodiment of the disclosure.

FIGS. 5A to 5E are graphs of Raman spectroscopy responses for each of aplurality of sand samples, Sands 1 through 5, before and after exposureto microwave electromagnetic radiation, according to an embodiment ofthe disclosure, and in particular, FIG. 5A is such Raman spectroscopyresponse of Sand 1, FIG. 5B is such Raman spectroscopy response of Sand2, FIG. 5C is such Raman spectroscopy response of Sand 3, FIG. 5D issuch Raman spectroscopy response of Sand 4, and FIG. 5E is such Ramanspectroscopy response of Sand 5.

FIG. 6 is a tumbling apparatus that may be used to tumble washed silicasand after exposure to microwave electromagnetic radiation according toan embodiment of the disclosure.

FIG. 7A is a scanning-electron-microscope enlarged image of angular sandbefore undergoing a tumbling process according to an embodiment of thedisclosure.

FIG. 7B is a scanning-electron-microscope enlarged image of the angularsand after undergoing a tumbling process according to an embodiment ofthe disclosure.

FIG. 8A is a schematic flow diagram illustrating an embodiment of anarrangement to strengthen sand proppant, which includes a sand screeningprocess, an attrition process, a drying process, a microwaving process,and a tumbling process.

FIG. 8B is a schematic flow diagram illustrating another embodiment ofan arrangement to strengthen sand proppant, which includes an attritionprocess, a drying process, a microwaving process, and a tumblingprocess.

FIG. 9 is a schematic flow diagram illustrating an embodiment of theattrition process of FIGS. 8A and 8B.

FIG. 10 is a schematic flow diagram illustrating an embodiment of themicrowaving process of FIGS. 8A and 8B.

FIG. 11 is a schematic flow diagram illustrating an embodiment of thetumbling process of FIGS. 8A and 8B.

FIG. 12 is a schematic diagram of an attrition process, a microwavingprocess, and a tumbling process according to one or more embodiments ofthe disclosure further having a mobile or at least semi-mobilearrangement.

DETAILED DESCRIPTION

So that the manner in which the features and advantages of theembodiments of the systems and methods disclosed herein, as well asothers, which will become apparent, may be understood in more detail, amore particular description of embodiments of systems and methodsbriefly summarized above may be had by reference to the followingdetailed description of embodiments thereof, in which one or more arefurther illustrated in the appended drawings, which form a part of thisspecification. It is to be noted, however, that the drawings illustrateonly various embodiments of the embodiments of the systems and methodsdisclosed herein and are therefore not to be considered limiting of thescope of the systems and methods disclosed herein as it may includeother effective embodiments as well.

In one or more embodiments, systems and methods of increasing crushresistance of silica sand, e.g., sand proppant used in well fracking,are disclosed. Further, in one or more embodiments, systems and methodsof enhancing the fracture conductivity of silica sand are disclosed.Such systems and methods strengthen the silica sand particle through acrystallography change and/or by making sand grains more round throughone or more of the processes of attrition, radiation exposure, andtumbling.

With extremely high quality white sand, such as those found in the upperMidwest of the United States, the weight percent of silicon dioxide(SiO₂) to the total weight of the sand may be as high as about 99.5%.For sands with lower quartz content than typical northern white sandsfound in the St. Peters or Jordan formations in the Northern UnitedStates, such as west Texas brown sand, however, the weight percent ofsilicon dioxide may be more on the order of about 96%, with the balancebeing up to about 3% aluminum oxide (Al₂O₃) and up to about 1% othercarbonates and/or oxides, e.g., calcium carbonate (CaCO₃), magnesiumcarbonate (MgCO₃), iron oxide (Fe₂O₃), among others. Thus, depending onthe geographical origin of a particular sand, its weight percentcomposition may be from 96% to 98% SiO₂, 1% to 3% Al₂O₃, and theremaining approximately 1% being other oxides and/or carbonates.

Screening Process

Raw sand can be sorted through a screening process to divide the sand byparticle size ranges. Screened sands are classified by their particlesize ranges, e.g., 20/40, 40/60, etc., where the numbers identify themesh sizes and increasing mesh sizes represent smaller openings. Forexample, a 20/40 sand indicates that 90% of the sand particles thereinshould be retained by screens having mesh sizes between 20 and 40. Sandthat has been sorted by sand particle size has been found to improve theeffect of the attrition, microwaving and tumbling processes, describedin greater detail hereinafter, because the sorted sand particles areuniform in size range.

FIG. 1 shows an embodiment of a trommel barrel sand screening apparatus10 that may be used to sort sand by mesh size. The raw sand feed 16 isplaced into one end portion of the trommel barrel 20, which is rotatedby a motor 14 positioned to rotate the trommel barrel 20 at a variablespeed. As illustrated, motor 14 is positioned proximate the entrance endportion of the trommel barrel 20, both of which are supported by supportmember 11. The raw sand feed 16, upon entering the trammel barrel 20, isrotated and moved gradually downwardly through the trommel barrel 20 viagravity and assisted by the rotation. Openings 21 in a first section 22of the trommel barrel 20 are relatively small, such that sand fines passtherethrough and fall into a designated fines container 15. Larger sandgrains continue to move downwardly through the trommel barrel 20 andinto the next section 24 thereof. The openings 23 in this section 24 arelarger than the openings 21 of the previous section 22, such that largersand grains pass therethrough and into the designated container 17. Asshown, the openings 23 are sized at 40 mesh, which indicates that sandgrains smaller than 40 mesh will pass therethrough and into thecontainer 17. Larger sand grains continue to move downwardly through thetrommel barrel 20 and into the next section 26 thereof. The openings 25in this section 26 are larger than the openings 21, 23 of the previoussections 22, 24, such that larger sand grains pass therethrough and intothe designated container 19. As shown, the openings 25 are sized at 20mesh, which indicates that sand grains between 20 mesh and 40 mesh willpass therethrough and into the designated container 19. The sand ofcontainer 19 is thus 20/40 mesh sorted sand. Larger sand grains continueto move downwardly through the trommel barrel 20 and into subsequentsections (not shown) having openings of larger mesh size. Eventually,any remaining large grain sand particles flow out of an end portion 18of the trommel barrel 20 and into a designed oversize container 27.

Attrition Process

In one or more embodiments, an attrition process is first used toeliminate as much of the non-silicon dioxide impurities as possible. Theattrition process involves washing the sand with a wash solution, suchas water and/or an ammonium chloride (NH₄Cl) solution, to remove up toabout 3% Al₂O₃ and lesser amounts of CaCO₃, MgCO₃, and/or Fe₂O₃ from thesand, thereby leaving a greater concentration of silicon dioxide. Theammonium chloride solution may have a weight percent concentration ofNH₄Cl in water of between about 2% and about 20%, between about 5% andabout 15%, between about 7.5% and about 12.5%, or any percentage inbetween those ranges. The ammonium chloride solution is acidic and formsa mild hydrochloric acid. The acid effectively acid washes the sand,whereby the acid reacts with non-SiO₂ impurities to create water-solublecompounds that may be washed away and removed from the remaining silicondioxide. The effectiveness of the attrition process is measured bycomparison of chemical analyses of sand samples that have undergone theattrition process and those analogous sand samples that have not.

Although ammonium chloride is effectively used in the attrition process,those skilled in the art will recognize that other mild acids, e.g.,hydrochloric, phosphoric, etc. may be equally employed in place of or inaddition to the ammonium chloride solution. Additionally, atetrahydrofuran (C₄H₈O) solution may be used, if desired, as an organicremoval agent to remove any organics from the sand. Thus, the attritionprocess seeks to increase the percentage of silicon dioxide in the sandby removing any and all non-SiO₂ impurities whether organic orinorganic.

FIG. 2 illustrates an embodiment of an apparatus 30 that may be used inan attrition process. As shown, the sorted sand 19 is placed in a washer36, e.g., vessel, tank, holding cavity of a screw washer, etc., eitherby conveyor or by direct placement, such as by front end loader. Thewater and/or acid solution 42 is added to the washer 36 containing thesand 19 through piping. The water and/or acid solution 42 is subjectedto mild to slightly aggressive agitation by a mixing blade or otheragitator 32 for a period of about fifteen minutes. In one or moreembodiments, such agitation may be for a preselected time and/or be fora period of about 10 to about 20 minutes, for about 5 to about 25minutes or any period of time therebetween. Thereafter, the spent waterand/or acid solution 44 is decanted from the washer 36, e.g., via anoverflow drain. Optionally, fresh water is added to the washer and thesand/fresh water mixture is agitated to further wash the treated sand.After such fresh water rinse, the water is similarly decanted from thewasher 36. The sand is removed from the washer 36, e.g., by a screwseparator 38 rotated by a motor 46, which offloads the sand 34 onto aconveyor or into a holding vessel (not shown). In one or moreembodiments, the conveyer with the sand thereon may be passed through adryer where any remaining water/moisture in the sand may be evaporatedwith added heat. Alternatively, or additionally, the sand may bepermitted to air dry.

Example 1

In an example of the attrition process, 80 grams of sand were added to130 ml of a 10% ammonium chloride solution in a beaker. The sand andammonium chloride solution were mixed by a mechanical blade mixer at arate of 300 rpm for approximately 15 minutes. The ammonium chloridesolution was then decanted from the sand sediment in the beaker. Thesand was dried by exposure to air for about 8 hours. However, the dryingtime could have been reduced by subjecting the sand to temperatures ofbetween about 120° F. and about 135° F. As will be understood by thoseskilled in the art, tetrahydrofuran, for example, was not used duringthe attrition process to remove any organics present.

Five different sand samples and a control sample, as will be understoodby those skilled in the art, were treated through this attritionprocess: Sand 1 is 20/40 “brown sand” from west Texas; Sand 2 is 20/40low-quartz, angular sand obtained from Gansu Provence, China; Sand 3 isa 20/40 low-quartz, dune sand obtained from Michigan; Sand 4 is a 20/40high-quartz, “white sand” with fractures obtained from Ohio; and Sand 5is a 20/40 high-quartz, high strength “Jordon white sand.” The controlsand, labeled as Control, is a sample of the same high-quartz, highstrength “Jordan white sand” as Sand 5 that remained untreated by theattrition process. All sand samples were sieve matched to the same sievedistribution to avoid any biased due to sieve distribution. Sievedistribution was conducted according to ISO 13503-2 and API RP 19C.

TABLE 1 shows the percentage compositions of Sands 1 through 5 and theControl before Sands 1 through 5 were treated through the aforementionedattrition process.

TABLE 1 Sample SiO2 Al2O3 Fe2O3 Na2O K2O CaO MgO Organic # (%) (%) (%)(%) (%) (%) (%) (%) Sand 1 96.55 1.66 0.16 0.25 0.91 0.13 0.07 0.26 Sand2 96.44 1.82 0.18 0.29 0.94 0.06 0.08 0.18 Sand 3 96.59 1.75 0.18 0.280.86 0.06 0.08 0.19 Sand 4 98.46 1.04 0.02 0.00 0.01 0.15 0.19 0.13 Sand5 99.12 0.60 0.00 0.00 0.02 0.18 0.05 0.03 Control 99.12 0.60 0.00 0.000.02 0.18 0.05 0.03

TABLE 2 shows the percentage compositions of Sands 1 through 5 and theControl after Sands 1 through 5 were treated according to the exampleattrition procedure.

TABLE 2 Sample SiO2 Al2O3 Fe2O3 Na2O K2O CaO MgO Organic # (%) (%) (%)(%) (%) (%) (%) (%) Sand 1 97.4 1.54 0.21 0.12 0.34 0.05 0.02 0.32 Sand2 97.58 1.34 0.22 0.11 0.41 0.02 0.01 0.31 Sand 3 97.45 1.43 0.21 0.110.44 0.05 0.02 0.29 Sand 4 99.16 0.59 0.04 0.0 0.0 0.03 0.02 0.16 Sand 599.63 0.31 0.00 0.00 0.00 0.00 0.00 0.06 Control 99.12 0.60 0.00 0.000.02 0.18 0.05 0.03

As shown in TABLE 2, all five of the sands, Sands 1 through 5, had theirpercentage by weight of silicon dioxide increase as a result of theattrition process, relative to their respective pre-treatmentpercentages. Conversely, the relative percentages of oxide and carbonateimpurities decreased when pre- and post-treatment percentages arecompared for each of Sands 1 through 5. The relative increase in thepercentage of organics between pre- and post-treatment of Sands 1through 5 shows that the ammonium chloride solution had little if anyeffect on removal of the organic compounds present. Thus, treatment ofthe Sands 1 through 5 with tetrahydrofuran to remove the organics, whichwas not done in this Example 1, may further increase the percentage ofsilicon dioxide in the sands relative to remaining impurities. As wouldbe expected, the Control sand did not see any relative change inpercentage silicon dioxide or impurities because the Control sand didnot undergo the attrition process.

A comparison of TABLE 1 and TABLE 2 also shows that the disclosedattrition process retains much of the aluminum oxide, such that thepercentage of aluminum oxide is only slightly reduced as compared to theother oxides and carbonates. An attrition process that can retainaluminum oxide (Al₂O₃), but eliminate other non-silicon dioxideimpurities, may have a synergistic strengthening effect on the silicasand during the microwaving process. Aluminum oxide is oxygen rich and,during the microwaving process, may share oxygen atoms with siliconatoms to achieve an enhanced alpha quartz or alpha quartz-likestructure. Aluminum oxide also adds to the density of, and impartsbauxite-like strength to, silica. For example, sintered bauxite, orcorundum (crystalline aluminum oxide), has a Mohs scale hardness of 9whereas that of quartz is 7.

Microwave Process

In one or more embodiments, a microwaving process is used to expose thequartz in the silica sand to microwave radiation in the frequency rangeof between 300 MHz and 300 GHz; with wavelengths ranging from one meterto one millimeter. After the attrition process, the silica sand isexposed to microwaves having frequencies and wavelengths in theaforementioned range. In one or more embodiments, the microwaves mayhave a frequency and wavelength in a tighter range, e.g., a frequencyfrom between about 1200 MHz to about 2700 MHz; with a wavelength frombetween about 110 mm to about 250 mm. The silica sand is exposed to themicrowave radiation for a period of time, e.g., a preselected time,ranging from about 2 minutes to about 5 hours. In one or more otherembodiments, the exposure time is between about 1 minute to about 10minutes, from about 2 minutes to about 8 minutes, or from about 3minutes to about 6 minutes. Beneficial results have been witnessed byApplicants with exposure times of as little as 30 to 45 seconds,however.

FIG. 3 illustrates one embodiment of a microwave device 50 that may beused to expose sand to microwave electromagnetic radiation. Sand isplaced onto a conveyor 61 that is arranged to move into and out of aninterior heating chamber or applicator 58 defined by an outer housing.The conveyor 61 is designed to move internally within the heatingchamber 58 past a magnetron (not shown), a waveguide 54 of a microwavegenerator 52, or other local source of microwave electromagneticradiation (not shown). The heating chamber or applicator 58 is arrangedto contain and distribute the microwave electromagnetic radiation aroundthe sand positioned on the conveyor 61. Thus, sand placed on theconveyor 61 should be spread out such that the thickness of the sandthereon is only a few grains thick, and in at least some embodiments,only one to two grains thick. A sand placement that is too thick mayobscure microwave electromagnetic radiation from penetrating and/orexposing grains of sand closer to the conveyor 61. In one or moreembodiments, the conveyor 61 is moved at a slow rate so that each sandgrain remains within the heating chamber 58 and is exposed to themicrowave electromagnetic radiation emitted from the waveguide 54 orother microwave energy source for a microwave exposure time of betweenabout 3 and about 5 minutes. The microwaving device 50 of FIG. 3 permitssand to be exposed to microwave electromagnetic radiation on acontinuous or semi-batch basis. The frequency and wavelength of themicrowave electromagnetic radiation may be selected to be within any ofthe respective ranges disclosed herein. As will be recognized by thoseskilled in the art, FIG. 3 also illustrates two chokes 64, which aremicrowave suppression tunnels to reduce microwave leakage from theheating chamber 58 via the conveyer 61. A circulator or isolator 56 isshown disposed in the wave guide 54 proximate the microwave generator 52to protect the magnetron (not shown) in the microwave generator 52 fromexcessive levels of reflected microwave electromagnetic radiation. Awaterload 62 is also positioned within device 50 to absorb microwaveelectromagnetic radiation into a high loss medium, e.g., water.

FIG. 4 illustrates another embodiment of a microwave device 70 that maybe used to expose sand to microwave electromagnetic radiation. Themicrowaving device includes a housing 76 having an access door 78 in aside wall thereof that permits access to a chamber (not shown) definedby the housing 76. Sand is placed directly into the chamber via theaccess door 78 and spread out on an inner horizontal surface thereofsuch that the sand layer thickness is only a few sand grains thick,e.g., 1 to 2 sand grains thick. Alternatively, the sand may be placed ona carrier device (not shown) that permits easy insertion of the carrierdevice into the chamber via the access door 78 and removal of thecarrier device after the microwave process has been completed. Amicrowave generator 72, magnetron (not shown), or other local devicecapable of emitting microwave electromagnetic radiation is positioned inthe housing 76 (embodiment not shown), or via a waveguide 74, to exposethe sand inside the chamber to emitted microwave electromagneticradiation. The microwaving device 70 of FIG. 4 permits sand to beexposed to microwave electromagnetic radiation on a batch basis. Likethe embodiment disclosed with respect to FIG. 3, the frequency andwavelength of the microwave electromagnetic radiation may be selected tobe within any of the respective ranges disclosed herein.

Through the microwaving process, a transformation in the sand particles'quartz crystallography occurs that strengthens the sand particles andmay also smooth the surface of the particles. The microwave radiationimparts energy to the sand particles that may reduce bonding between thesilicon dioxide molecules and any impurities such that a more homogenoussilicon dioxide structure may result upon cooling. Having adequateoxygen in the air surrounding the sand during the microwaving processmay facilitate the improved bonding of the silicon dioxide molecules.

Often, silica sands that are used in hydraulic fracturing have aspecific gravity that ranges from about 2.45 to about 2.54. Poorerquality sands have specific gravities below about 2.50. The specificgravity of silica sand treated via the attrition and microwavingprocesses disclosed herein have been shown to increase as a result ofsuch treatments. TABLE 3 provides the initial specific gravity (prior toundergoing the microwaving process) and the final specific gravity(after undergoing the microwaving process) for each of Sands 1 through 5and the Control sand.

TABLE 3 Sample Type Initial specific gravity Final specific gravity Sand1 2.48 2.54 Sand 2 2.45 2.50 Sand 3 2.46 2.51 Sand 4 2.48 2.53 Sand 52.53 2.61 Control 2.54 2.54

Analyzing the data in TABLE 3, each of the Sands 1 through 5 had itsspecific gravity increased by undergoing the microwaving processdisclosed herein. In fact, the specific gravity of each of Sands 1through 5 increased to be at or above about 2.50, with the range beingfrom about 2.50 to about 2.61. For Sand 5, its specific gravityincreased to slightly above 2.60. A specific gravity of 2.60 correspondsto a nearly 100% alpha quartz crystallography. Thus, for each of theSands 1 through 5, the microwaving process increased the specificgravity by over about 2% and toward that indicative of a true alphaquartz crystallography. As would be expected, however, the initial andfinal specific gravities of the Control sand remained the same, becausethe Control sand was not exposed to microwave electromagnetic radiation.

The depth of the transformation from the surface of the quartz sandparticles exposed to the microwave radiation appears to correlate withthe length of exposure. Thus, for a given frequency, longer microwaveexposure times may cause a deeper transformation of the quartz sandparticles than do shorter microwave exposure times. Therefore, anoptimum exposure time may be sought which balances the economics ofexposure time and the benefits imparted to the quartz sand. Anotherconsideration is selection of an embodiment of a microwave device orsystem arranged and designed to uniformly expose the sand grainparticles to the microwave electromagnetic radiation, thereby permittinguniform heating.

In some arrangements, the exposure time can be reduced significantly,while maintaining the same or greater strengthening of the quartz sandparticles, if the frequency and wavelength ranges are targeted between 8GHz to 75 GHz and 4.0 mm to 37.5 mm, respectively. However, microwavegenerators, e.g., magnetrons, capable of emitting microwaves withinthese frequency and wavelength ranges tend to be very expensive, andthus may not be commercially viable sources for the microwaving process.

Example 2

The crush resistance of the five different sands described above wasdetermined prior to subjecting each type of sand to the microwavingprocess. Accordingly, samples of the five different sands, each havingundergone the attrition process disclosed above, and the Control sandwere first tested for crush resistance on a dry basis following theprocedures generally set out in API RP 56, which prescribe standard,recommended practices for testing sand used in gravel packingoperations. For each of the Sands 1 through 5 and the Control sand, 40grams of the particular sand was placed into a cylindrically-shapedcrush cell. A piston, arranged to fit within the cylindrically-shapedcrush cell without significant gap therebetween, was lowered into thecrush cell above the sand sample. The piston was moved downwardly toapply pressure on the sand sample disposed at the bottom of the crushcell. In conducting this dry crush test, one minute was permitted forthe piston to attain the desired pressure against the sand sample. Afterthe initial minute, the desired pressure was maintained for anadditional two minutes.

In a first set of tests, each of the sand samples was subjected to 6,000psi pressure by the piston. Each sample was then placed on a 40 meshscreen to determine how many fines had been created by thepiston-applied 6,000 psi. The percentage of the total sand fallingthrough the 40 mesh screen and onto the pan beneath, i.e., fines lessthan 40 mesh, represents the crush percentage for each sand sample. Theabove procedures were repeated for a second set of samples of Sands 1through 5 for a piston-applied pressure of 8,000 psi.

TABLE 4 provides the pre-microwave process crush data for each of thesand samples, Sand 1 through Sand 5, and the Control sand, from thesetwo separate crush tests: one conducted at 6,000 psi and the otherconducted at 8,000 psi. The 6,000 psi crush test data is provided on thesecond column of Table 4, and the 8,000 psi crush test is provided onthe third column thereof.

TABLE 4 Sample # % crush at 6,000 psi % crush at 8,000 psi Sand 1 7.8413.95 Sand 2 8.68 15.46 Sand 3 8.56 14.74 Sand 4 11.62 27.19 Sand 5 5.6910.08 Control 5.69 10.08

Samples of the five sands, Sands 1 through 5, (but not the Control sand)were then separately exposed to microwave radiation in a lab scalemicrowave device. The lab scale microwave device that was used has amicrowave/heating chamber with a Plexiglas side or front. The microwavechamber is similar to that shown with respect to FIGS. 3 and 10 and hasa length of about three feet. A 14-inch wide conveyor belt runs throughthe microwave chamber and is operated by a variable speed drive. Sandparticles were spread upon the top conveyor belt proximate the entranceto the microwave chamber. The variable speed drive controlled the speedof the conveyor belt such that the individual sand particles wereexposed to approximately 120 seconds of microwave electromagneticradiation in the microwave chamber. The microwave electromagneticradiation had a frequency of about 1665 MHz and a wavelength of about180 mm. As the microwave treated sand particles exited the microwavechamber, they fell off of the end portion of the conveyor and into atreated sand collection container.

After exposing each of the Sands 1 through 5 to the microwave radiationas specified above in this Example 2, the crush resistance of these fivedifferent sands and the Control sand (which was not exposed to microwaveradiation) was determined using the same procedure described above.

TABLE 5 provides the post-microwave process crush data for each of thesand samples, Sand 1 through Sand 5, and the Control sand, from the twoseparate crush tests: one conducted at 6,000 psi (second column) and theother conducted at 8,000 psi (third column).

TABLE 5 Sample # % crush at 6,000 psi % crush at 8,000 psi Sand 1 6.5211.85 Sand 2 7.46 13.48 Sand 3 7.25 12.62 Sand 4 10.82 25.29 Sand 5 3.887.88 Control 5.69 10.08

Comparing the crush data of TABLE 4 and TABLE 5, each of themicrowave-treated Sands 1 through 5 saw a decrease in the percentage ofcrush, or fines less than mesh size 40, resulting from the pressuretests performed. This result was realized for both of the pressure testsat 6,000 psi and at 8,000 psi. Looking at the Control sand between TABLE4 and TABLE 5, no difference in the percentage of crush resulted. Thiswas to be expected since neither Control sand sample was exposed tomicrowave electromagnetic radiation.

Example 3

In another test designed to account for the heat and moisture that maybe experienced by sand in a downhole well, the crush resistance ofsamples of each of the five different sands, Sands 1 through Sand 5, andthe Control sand was tested in hot and wet conditions both prior tomicrowave electromagnetic radiation treatment and afterward. Each ofsand samples, Sand 1 through Sand 5, first underwent the attritionprocess disclosed above. Although there may be no standard testprocedure for hot-wet crush, the procedures for carrying out this crushtest of Example 3 were the same as those described for Example 2. Thecrush cell was modified, however, to permit fluid to be present duringthe crush test. This modification included making entry and exitopenings in the crush cell to allow a 2 wt % KCl solution at about 210°F. to flow through the sand sample in the crush cell throughout theapproximate three minute duration of the hot-wet crush test.Additionally, the crush cell was pre-heated to approximately 250° C.prior to adding the sand sample and the KCl solution water. In thishot-wet crush test, each 40 gram sample of the Sands 1 through 5 and theControl sand was added to the modified crush cell. The proceduresaccording Example 2 then were followed to test each sand sample and the2 wt % KCl solution at about 210° F. was pump through the modified crushcell during the approximate 3 minute test. Fines were collected aftereach test and dried in a pan prior to measuring the percentage of finesgenerated as a result of each test.

TABLE 6 provides the pre-microwave process hot-wet crush data for eachof the dried sand samples, Sand 1 through Sand 5 and the Control sand,from these two separate hot-wet crush tests: one conducted at 6,000 psiand the other conducted at 8,000 psi. The 6,000 psi crush hot-wet testdata is provided on the second column of Table 6, and the 8,000 psihot-wet crush test is provided on the third column of Table 6. For eachof the Sands 1 through 5 and the Control sand, three samples were testedthrough the same procedure and the results averaged to arrive at thedata presented in Table 6.

TABLE 6 Sample # % crush at 6,000 psi % crush at 8,000 psi Sand 1 10.0616.97 Sand 2 12.95 20.05 Sand 3 11.42 18.52 Sand 4 15.94 32.86 Sand 58.69 14.43 Control 8.69 14.43

Samples of the five sands, Sands 1 through 5, (but not the Control sand)were then separately exposed to microwave electromagnetic radiation. Thesame procedures and devices used with respect to the samples of Example2 were also used to expose the sand samples to magnetic electromagneticradiation in this Example 3.

After exposing each of the Sands 1 through 5 to the microwave radiationas specified, the hot-wet crush resistance of these five different sandsand the Control sand was determined using the same procedure describedabove.

TABLE 7 provides the post-microwave process hot-wet crush data for eachof the dried sand samples, Sand 1 through Sand 5, and the Control sand,from the two separate hot-wet crush tests: one conducted at 6,000 psi(second column) and the other conducted at 8,000 psi (third column). Foreach of the Sands 1 through 5 and the Control sand, three samples weretested through the same procedure and the results averaged to arrive atthe data presented in Table 7.

TABLE 7 Sample # % crush at 6,000 psi % crush at 8,000 psi Sand 1 8.0612.16 Sand 2 9.31 15.05 Sand 3 8.82 15.66 Sand 4 13.68 29.96 Sand 5 6.5211.35 Control 8.69 14.43

Comparing the crush data of TABLE 6 and TABLE 7, each of themicrowave-treated Sands 1 through 5 saw a decrease in the percentage ofcrush, or fines less than mesh size 40, resulting from the pressuretests performed. This result was realized both for the pressure tests at6,000 psi and at 8,000 psi. Looking at the Control sand between TABLE 6and TABLE 7, no difference in the percentage of crush resulted. This wasto be expected since neither Control sand sample was exposed tomicrowave electromagnetic radiation.

Example 4

A more comprehensive sieve distribution analysis was conducted of theeach of the sand samples, Sands 1 through 5, and the Control sand,undergoing the hot-wet crush test according to Example 3. The sievedistribution was conducted according to ISO 13503-2 and API RP 19C, aswill be understood by those skilled in the art. TABLE 8 provides thepercentage by sieve number of the total composition of each of the sandsamples, Sands 1 through 5, and the Control sand, after undergoing thehot-wet crush test according to the procedures of Example 3. The data inTABLE 8 was measured from sand samples that were not exposed tomicrowave electromagnetic radiation in the microwaving process disclosedherein.

TABLE 8 Sieve Sand 1 Sand 2 Sand 3 Sand 4 Sand 5 Control 30 0.00 0.000.00 0.00 1.03 1.07 40 0.00 0.00 0.00 0.00 1.31 1.69 50 2.62 2.07 2.921.62 4.19 3.62 60 5.81 4.42 3.59 4.16 8.26 8.18 70 15.27 5.24 6.22 7.5220.06 19.28 100  28.30 31.72 25.41 9.88 23.94 25.72 140  3.19 5.82 13.317.80 4.30 4.16 Pan 44.81 50.73 48.55 69.02 36.91 36.28

TABLE 9 provides the percentage by sieve number of the total compositionof each of the sand samples, Sands 1 through 5, and the Control sand,after undergoing the hot-wet crush test according to the procedures ofExample 3. The data in TABLE 9 were measured from sand samples that wereexposed to microwave electromagnetic radiation in the microwavingprocess as disclosed with respect to Example 2.

TABLE 9 Sieve Sand 1 Sand 2 Sand 3 Sand 4 Sand 5 Control 30 0.00 0.000.00 0.00 1.87 1.11 40 0.55 0.00 0.00 0.00 2.26 1.82 50 3.13 2.12 3.130.00 5.16 3.57 60 6.35 4.65 4.06 6.03 10.05 8.35 70 17.38 8.67 7.95 7.7823.64 20.52 100  32.49 35.44 29.17 13.19 27.66 26.19 140  20.95 20.6626.11 28.15 18.34 3.03 Pan 19.15 28.46 29.58 44.85 11.02 36.52

Comparing the results between TABLE 8 and TABLE 9, the sand samples,Sands 1 through 5, that were exposed to microwave electromagneticradiation, as represented by the data of TABLE 9, generally show a muchgreater overall percentage of sand grain particles that fall withinmeasureable sieve sizes, e.g., 50, 60, 70, 100, and 140, than theanalogous sand samples that did not undergo the microwaving process.Conversely, the sand samples, Sands 1 through 5, that were not exposedto microwave electromagnetic radiation, as represented by the data ofTABLE 8, generally show a much greater overall percentage of unmeasuredsieve size fines, identified as “pan” (as understood by those skilled inthe art), than the analogous sand samples that underwent the microwavingprocess.

Fracture conductivity testing may be used to assess the effectiveness ofthe microwaving process at increasing the strength, quality anddurability of sand proppant. Fracture conductivity testing assesses howa proppant will perform in the wellbore once injected into hydraulicfractures within the formation. Fracture conductivity is thus ameasurement of how easily a fluid flows through a fracture and iscalculated as the permeability multiplied by the thickness of theproppant pack according to the following equation:

kW _(f) =μQL/wΔP

where k is the proppant pack permeability in darcy, W_(f) is the packthickness in cm, μ is the viscosity of the test liquid at roomtemperature in cp, Q is the flow rate in cm³/s, L is the length betweenpressure ports in cm, w is the width of the cell in cm, and ΔP is thepressure drop (P_(upstream)−P_(downstream)) in kPa.

Silica sand proppants having a greater crush resistance tend toeffectuate greater fracture conductivities as compared to silica sandproppants having lower crush resistance. Sands of lower crushresistance, once subjected to high pressure in the hydraulic fractures,may create greater amounts of fines that interfere with and hinder theconductivity of fluid passing through the fractures, thereby decreasingthe conductivity.

Example 5

To understand the expected differences in the fracture conductivities ofthe silica sand samples, Sands 1 through 5, with and without microwaveelectromagnetic radiation treatment, the following standard conductivityprocedures were employed: API RP 19D (Recommended Practice for Measuringthe Long-term Conductivity of Proppants, 2008) and ISO13503-5:2006(standard testing procedures for evaluation proppants used in hydraulicfracturing and gravel packing operations), as will be understood bythose skilled in the art.

It should be noted that Sands 1 through 5 and the Control sand were eachtested in separate tests. Under the API RP 19D Standard testing, a Cookecell or testing cell, having pressure ports spaced along its length, wasset up according to the Standard's procedures. The Cooke cell has a topand bottom piston that applies pressure stresses to simulate formationpressures that may be encountered by the proppant in the wellbore.

For each test, the sand sample was loaded into the core of the Cookecell between sandstone platens and at an equivalent of about two poundsof sand per square foot. The top and bottom pistons apply pressure at arate of 100 psi/min+/−5 psi/min until the cell has reached a 2,000 psiincrement, at which point the 2,000 psi pressure is maintained forapproximately 50 hours. Because each pressure interval requires 50 hoursof time to reach a steady state prior to any measurement, theconductivity test may take two to three weeks to complete. A 2 wt %potassium chloride (KCl) solution was then pumped into the Cooke cellthrough an end thereof at a rate between 2 ml/min and 8 ml/min to reduceany clay swelling and thereby mitigate variability between the tests ofthe several samples. The KCl solution and Cooke cell were maintained at150° F. throughout the testing. Data were measured a minimum of fivetimes at each pressure increment and averaged to yield a permeabilityfor each pressure. According to the API RP 19D Standard, each sample ofsand is tested at pressure increments of 2,000 psi, 4,000 psi and 6,000psi. In this Example 5, a pressure interval of 8,000 psi also wastested.

TABLE 10 shows the pre-microwave process proppant conductivity data foreach of the sand samples, Sand 1 through Sand 5, and the Control sand,using the fracture conductivity tests described above. The conductivityat 6,000 psi is given in the second column for each sample and theconductivity at 8,000 psi is given in the third column for each sample.

TABLE 10 Sample Conductivity (md-ft) Conductivity (md-ft) # at 6,000 psiat 8,000 psi Sand 1 1110 557 Sand 2 820 480 Sand 3 981 447 Sand 4 628125 Sand 5 1396 595 Control 1428 613

TABLE 11 provides the proppant conductivity data, using the fractureconductivity tests described above, for each of the sand samples, Sand 1through Sand 5 and the Control sand, after each of the sand samples(except the Control sand sample) was exposed to microwaveelectromagnetic radiation during the microwaving process disclosed abovein Example 2. The conductivity at 6,000 psi is given in the secondcolumn for each sample and the conductivity at 8,000 psi is given in thethird column for each sample.

TABLE 11 Proppant Conductivity after treatment Sample Conductivity(md-ft) Conductivity (md-ft) # at 6,000 psi at 8,000 psi Sand 1 1465 700Sand 2 1010 601 Sand 3 1240 587 Sand 4 775 188 Sand 5 1654 779 Control1391 607

When comparing the proppant conductivity data of TABLE 10 and TABLE 11,each of the microwave-treated Sands 1 through 5 shows a marked increasein the conductivity (TABLE 11) based on the pressure tests performed ascompared to its analogous sand sample that was not treated withmicrowave electromagnetic radiation (TABLE 10). This result was realizedboth for the pressure tests at 6,000 psi and at 8,000 psi. Looking atthe Control sand between TABLE 10 and TABLE 11, the conductivities atthe 6,000 psi pressure interval and the 8,000 psi pressure intervalsremained about the same. This was to be expected since neither Controlsand sample was exposed to microwave electromagnetic radiation.

FIGS. 5A through 5E are graphical representations of Raman spectroscopyresponses, one graph for each of the sand samples, with responses beforeand after exposure to microwave electromagnetic radiation plotted. FIG.5A is the Raman spectroscopy response of Sand 1, FIG. 5B is the Ramanspectroscopy response of Sand 2, FIG. 5C is the Raman spectroscopyresponse of Sand 3, FIG. 5D is the Raman spectroscopy response of Sand4, and FIG. 5E is the Raman spectroscopy response of Sand 5. The x-axison each of FIGS. 5A to 5E shows the frequency range used for the test.Separate tests were run under low frequency range (from about 200 cm⁻¹to about 800 cm⁻¹) and high frequency range (from about 800 cm⁻¹ toabout 1600 cm⁻¹). Looking at the red plot showing the Raman spectroscopyresponse of the microwave-treated silica sand in FIGS. 5A through 5E,generally, there is a significant depression in intensity peaks around450 cm⁻¹, followed by an increase in intensity between about 950 to 1050cm⁻¹, followed by the disappearance or significant decrease in intensitypeaks after 1050 cm⁻¹, as compared to the blue plot showing the Ramanspectroscopy response of the untreated silica sand. These changes areindicative of structural changes in the quartz particles of the silicasand. The low frequency region of the Raman spectroscopy response plotsis less than about 800 cm⁻¹. The frequency near about 450 cm⁻¹ of thislow frequency region is associated with responses to mono- and di-valentions bonded to the silica structure, and is thus indicative of weaknessin the silica structure by invasion of impurities into the Si—O—Silattice. Close to perfect alpha quartz structure and bonding resonatesat a maximum intensity of around 1025 cm⁻¹, while responses in theregion between 950 to 1050 cm⁻¹ is still considered to be indicative ofthe presence of a strong, silica lattice with possibly some impurities.The distortion or shift from about 1025 cm⁻¹ indicates the presence ofan impurity which is sufficiently insignificant, for purpose of theRaman spectroscopy response, to place the resonance out of the “silica”region, i.e., region indicative of close to perfect alpha quartzstructure. The impurity, however, is still powerful enough to shift theresponse away at about 1025 cm⁻¹.

Tumbling Process

In one or more embodiments, a tumbling process is used to physicallyalter the shape of the individual sand grains. Thus, the tumblingprocess aims to eliminate or at least lessen the angularity of theindividual sand grains to thereby increase the roundness and/orsphericity of the sand grains. Most all sand, including evenhigh-quality white sand, can have outer surfaces with physicalcharacteristics of platy, layery, and scaly nature, along with manyridges and valleys present, that can be made more round and/or sphericalthrough a physical tumbling process.

In one or more embodiments, the tumbling process occurs almostimmediately, or without any significant delay, after the microwavingprocess. Referring to FIG. 6, the treated silica sand is placed in thebarrel 92 of a mixer or tumbler 90 that is capable of being rotated uponan axis thereof at least at about five rotations per minute and up toabout twenty rotations per minute. The barrel 92 has a lid or top 94that covers the barrel 92 during operation. An agitator or stirrer 96 isshown coupled to the inner lid or top 94 and may be arranged to agitateor stir the sand within the barrel 92 during operation. The tumbler ormixer 90 of one or more embodiments may be similar to that of FIG. 6,which is an Eirich mixer manufactured by Eirich Machines Inc. of Gurnee,Ill., as will be understood by those skilled in the art. Tumbling themicrowave-treated sand particles together in a tumbler for about 30minutes to about one hour increases the roundness of the sand particles.In one or more embodiments, the tumbling time may be preselected and maybe as little as about 5 minutes, about 10 minutes, about 15 minutes,about 20 minutes or about 25 minutes or as much as about 75 minutes,about 90 minutes, about 105 minutes or about 120 minutes. Although thetumbling process can be performed at any time, better results may beachieved if the tumbling process of the microwave-treated silica sand isstarted almost immediately following the microwaving process, e.g.,while the individual sand grains have a temperature above ambienttemperature. In one or more embodiments, the tumbling process is startedon the microwave-treated silica sand within about 30 seconds to about 1minute following the microwaving process. In one or more otherembodiments, the tumbling process is started on the microwave-treatedsilica sand within about 1 to about 2 minutes following the microwavingprocess. Yet still, in one or more other embodiments, themicrowave-treated sand is moved from the microwaving process to thetumbling process within about 5 minutes and the tumbling process begun.The microwaving process imparts heat energy to the individual silicasand grains; therefore, the sand grains are more malleable and moreeasily rounded soon after the microwaving process, e.g., within minutesthereof.

Additionally, the mixer or tumbler 90 shown in FIG. 6 can be modifiedwith heat bands (not shown) to raise the temperature of the sand and/ormaintain the sand temperature during the tumbling process. A temperaturerange of between about 125° F. to about 180° F., for example, may beused to facilitate altering the silica sand grains to be more roundand/or spherical in a shorter period of time. Insulation material (notshown) also may be added to the outer surface of the mixer to minimizeloss of heat. Although an Eirich mixer is shown in FIG. 6 as anembodiment of a mixer or tumbler, those skilled in the art willrecognize that a different type of mixer or tumbler may be employed solong as the sand grains are sufficiently agitated as to cause them tobecome more round and/or spherical.

FIG. 7A shows an enlarged image from a scanning electron microscope (aswill be understood by those skilled in the art) and illustrates theangularity of individual silica sand particles prior to undergoing thetumbling process according to one or more embodiments disclosed herein.FIG. 7B shows an enlarged image from a scanning electron microscope (aswill be understood by those skilled in the art) and illustrates theangularity of individual silica sand particles after the microwavetreated silica sand was tumbled for 30 minutes in at Eirich mixer usedfor foundry sand mold making. A quick comparison of the silica sandparticles between FIG. 7A and FIG. 7B clearly shows that silica sandparticles shown in FIG. 7B, which underwent the tumbling process arerounder and more spherical than those of FIG. 7A. Visual imagery, suchas the scanning electron microscope images of FIGS. 7A and 7B, is one ofthe more effective verifications of the improvements in the roundnessand/or sphericity to the individual silica sand particles imparted bythe tumbling process.

TABLE 12 provides the proppant conductivity data, using the fractureconductivity tests described previously, for each of the sand samples,Sand 1 through Sand 5 and the Control sand, after each of the sandsamples was treated in the microwaving process but before undergoing thetumbling process. The conductivity at 1,000 psi is given in the secondcolumn for each sample, the conductivity at 2,000 psi is given in thethird column for each sample, and the conductivity at 3,000 psi is givenin the fourth column of each sample.

TABLE 12 Sample Conductivity (md-ft) Conductivity (md-ft) Conductivity(md-ft) # at 1000 psi at 2000 psi at 3000 psi Sand 1 5505 4325 3746 Sand2 4333 3412 2757 Sand 3 4512 3735 2813 Sand 4 3916 3019 2275 Sand 5 55394404 3854 Control 5491 4422 3833

TABLE 13 provides the proppant conductivity data, using the fractureconductivity tests described previously, for each of the sand samples,Sand 1 through Sand 5 and the Control sand, after each of the sandsamples (except the Control sand) was tumbled during the tumblingprocess disclosed above. The conductivity at 1,000 psi is given in thesecond column for each sample, the conductivity at 2,000 psi is given inthe third column for each sample, and the conductivity at 3,000 psi isgiven in the fourth column of each sample.

TABLE 13 Sample Conductivity (md-ft) Conductivity (md-ft) Conductivity(md-ft) # at 1000 psi at 2000 psi at 3000 psi Sand 1 5762 4531 3881 Sand2 4813 3834 3054 Sand 3 4963 4122 3116 Sand 4 4121 3216 2475 Sand 5 58394676 4018 Control 5502 4437 3798

When comparing the proppant conductivity data of TABLE 12 and TABLE 13,each of the tumbled Sands 1 through 5 shows a marked increase in theconductivity (TABLE 13) based on the pressure tests performed ascompared to its analogous sand sample that was not tumbled (TABLE 12).This result was realized for the pressure tests at 1,000 psi, 2,000 psiand at 3,000 psi. Looking at the Control sand between TABLE 12 and TABLE13, the conductivities at each of the pressure intervals remained aboutthe same. This was to be expected since neither Control sand sampleunderwent the tumbling process.

Various embodiments of process arrangements to strengthen sand proppantswill now be described with reference to FIGS. 8A through 12. FIG. 8A isa schematic flow diagram illustrating an embodiment of an arrangement tostrengthen sand proppant, which includes a sand screening process, anattrition process, a drying process, a microwaving process, and atumbling process. In the arrangement 100 of FIG. 8A, raw sand 16 is fedinto the sand screening process, represented by block 110, and sorted bymesh size. The sorted sand 15, 17, 27, below the sand screening processblock 110, represents the raw sand 16 that was screened out through thisprocess. The sand sorted for further processing, e.g., by a sandscreening process similar to that shown and described with respect toFIG. 1, is routed to the attrition process represented by block 130. Awash solution, e.g., a KCl solution, is added and the spent washsolution is removed via line 44. The washed sand is sent to be dried asrepresented by block 140. Block 140 is optional and represents either aresidence time for air drying or a dryer in which the washed sand issubjected to heated air. The dried sand (or sand from block 130 if nodrying process is present) is then routed to the microwaving processrepresented by block 150. A waveform 54 is positioned to route microwaveelectromagnetic radiation into the microwaving process. Themicrowave-treated sand is then routed to a tumbling process, which isrepresented by block 190. After undergoing the tumbling process, thefinished sand 198 is stockpiled for subsequent use.

FIG. 8B is a schematic flow diagram illustrating another embodiment ofan arrangement to strengthen sand proppant, which includes an attritionprocess, a drying process, a microwaving process, and a tumblingprocess. The arrangement 101 of FIG. 8B is similar to the arrangement ofFIG. 8A, except that in the arrangement of FIG. 8A, the raw sand,represented by block 19, is pre-sorted. In one or more embodiments (notshown in FIG. 8A or 8B), the washed sand may be routed directly from theattrition process represented by block 130 to the microwaving processrepresented by block 150 without undergoing any drying process.

FIG. 9 is a schematic flow diagram illustrating an embodiment of theattrition process that is represented by block 130 of FIGS. 8A and 8B.As illustrated, sorted sand 19 enters the process at 47 via a conveyor31 supported by conveyor supports 33. The sand is offloaded into awasher 36 that has a wash solution therein and is suspended above floorlevel via supports 45. Fresh wash solution, e.g., a KCl solution, isadded via piping 42. The spent wash solution is decanted from the sandin the washer 36 via piping 44. The sand in the wash solution is stirredin the washer 36 by a mixing blade or agitator 32. A screw separator 38is rotated by a motor 46 and moves the sand upward along a wall of thewasher 36 and out of the wash solution. The washed sand 34 falls throughan outlet and onto a conveyor 35, which is supported by rollers 37. Theconveyor 35 moves the washed sand 34 into either a drying process ordirectly into the microwaving process. The attrition process is furtherdescribed above and with reference to FIG. 2 and Example 1.

FIG. 10 is a schematic flow diagram illustrating an embodiment of themicrowaving process that is represented by block 150 of FIGS. 8A and 8B.As illustrated, washed sand from block 130 (see FIG. 9) or from a dryingprocess (see FIG. 8A or 8B) enters the process at 51 via a conveyor 61supported by conveyor supports 63. The washed sand enters the heatingchamber or applicator 58 via the conveyor 61, which moves the washedsand at slow rate so that each sand grain remains within the heatingchamber or applicator 58 and is exposed to the microwave electromagneticradiation emitted from the waveguide 54 for a preselected microwaveexposure time, e.g., between about 3 and about 5 minutes. Mode stirrers66 are shown movably coupled to a top portion of the inner chamber 58and function to modify the electromagnetic boundary conditions withinthe heating chamber or applicator 58, which results in a temporalnon-stationary electric field pattern over the washed sand on conveyor61. Chokes 64 are also shown proximate to where the conveyor 61 entersand leaves the heating chamber or applicator 58. Chokes 64 are arrangedand act to minimize microwave electromagnetic radiation leakage frominside the heating chamber or applicator 58. The microwave-treated sand68 exits the heating chamber or applicator 58 via conveyor 61 at 69. Themicrowaving process is further described above and with reference toFIGS. 3 and 4 and Example 2.

FIG. 11 is a schematic flow diagram illustrating an embodiment of thetumbling process that is represented by block 190 of FIGS. 8A and 8B. Asillustrated, microwave-treated sand 68 enters the process at 91 via aconveyor 81 supported by conveyor supports 83. The sand 68 is offloadedinto a holding container 93. Sand 68 from the holding container 93 maythen be placed into one of a plurality of mixers or tumblers 90.Alternatively, the sand 68 may be offloaded directly into one of aplurality of mixers or tumblers 90. In one or more embodiments, thenumber of mixers or tumblers 90 is selected such that one mixer ortumbler 90 is being emptied of sand as another is being filled withsand. The number of mixers or tumbler needed for such semi-continuousoperation is dependent on the desired mixing/tumbling time of thetumbling process, as will be understood by those skilled in the art.Once the sand has tumbled for the desired period of time, the finishedsand 198 is offloaded into a storage or other container for use as sandproppant. The tumbling process is further described above and withreference to FIGS. 6, 7A, and 7B.

FIG. 12 is a schematic representation of the attrition process 230, themicrowaving process 250 and the tumbling process 290 according to one ormore embodiments of the disclosure further having a mobile or at leastsemi-mobile arrangement 200. In this embodiment, for example, a mobilesand attritioning and/or cleaning process 230 is provided as illustratedin a mobile trailer or mobile tank, a microwave or heating process 250is provided as illustrated in a mobile trailer or mobile tank (connectedto a tractor), and a tumbling process 290 is provided by a mobiletrailer or mobile tank (connected to a tractor). The sand treated in themobile sand attritioning process 230 may be conveyed via mobile conveyor235 to an inlet 234 of the mobile trailer or mobile tank providing themicrowave or heating process 250. The sand treated in the mobilemicrowave or heating process 250 may be conveyed via mobile conveyor 231to an inlet 268 of mobile trailer or mobile tank providing the tumblingprocess 290. The treated sand 298 may then be conveyed to sand stacks,sand unloading or piling areas, or a sand storage facility or locationby a conveyor 299 or other transporting source as will be understood bythose skilled in the art. The mobile trailers or tanks illustrated, aswill be understood by those skilled in the art, may include machinespositioned within or on a trailer and/or tank to include an attritioner236, a microwave heater 258, a microwave conveyor 261 and/or a tumbler292. This, for example, may allow the process to be positioned close toa rail spur or rail terminal located near a specific formation. Themobile or semi-mobile arrangement 200 then can be more readily moved toanother location where desired. Other locations, such as near a wellfracturing site or in a region where a plurality of well fracturingsites are located, also may be used as will be understood by thoseskilled in the art to reduce transportation distances of the treatedproppant or sand to a well fracturing or other site for usage, forexample.

This application is a continuation, and claims priority to and thebenefit of U.S. Non-Provisional patent application Ser. No. 16/276,910,filed Feb. 15, 2019, titled “SYSTEMS AND METHODS TO STRENGTHEN SANDPROPPANT,” which is a divisional of U.S. Non-Provisional Patentapplication Ser. No. 16/144,654, filed Sep. 27, 2018, titled “SYSTEMSAND METHODS TO STRENGTHEN SAND PROPPANT,” now U.S. Pat. No. 10,364,154,issued Jul. 30, 2019, which claims priority to and the benefit of U.S.Provisional Patent Application No. 62/690,073, filed Jun. 26, 2018,titled “SYSTEMS AND METHODS TO STRENGTHEN SAND PROPPANT,” the fulldisclosure of which is hereby incorporated herein by reference in itsentirety.

In the drawings and specification, several embodiments of systems andmethods to strengthen sand proppant have been disclosed, and althoughspecific terms are employed, the terms are used in a descriptive senseonly and not for purposes of limitation. Embodiments of systems andmethods have been described in considerable detail with specificreference to the illustrated embodiments. However, it will be apparentthat various modifications and changes can be made within the spirit andscope of the embodiments of systems and methods as described in theforegoing specification, and such modifications and changes are to beconsidered equivalents and part of this disclosure.

What is claimed is:
 1. A method of increasing crush resistance of silicasand proppant used in fracturing operations involving one or moresubterranean hydrocarbon formations, the method comprising: washingsilica sand proppant in a washer to remove impurities; exposing at leasta portion of the washed silica sand proppant to electromagneticradiation to impart heat to the washed silica sand proppant, theelectromagnetic radiation being generated by a local electromagneticradiation source; adding washed and exposed silica sand proppant to atumbler; and within about one minute after the at least a portion of thewashed silica sand proppant has been exposed to the electromagneticradiation, tumbling the at least a portion of washed silica sandproppant in the tumbler.
 2. The method of claim 1, further comprising:supplying the washed and exposed silica sand proppant, after tumbling,used in the fracturing operations involving one or more subterraneanhydrocarbon formations.
 3. The method of claim 1, wherein tumbling theat least a portion of washed silica sand proppant in the tumbler occurswithin about thirty seconds after that portion was exposed to theelectromagnetic radiation.
 4. The method of claim 1, wherein thetumbling occurs for at least about five minutes.
 5. The method of claim1, further comprising: contacting the silica sand proppant to an acidicsolution in the washer.
 6. The method of claim 5, wherein the acidicsolution in the washer also contains an organic removal agent.
 7. Themethod of claim 1, further comprising: contacting the silica sandproppant to an ammonium chloride solution in the washer.
 8. The methodof claim 1, wherein exposing the at least a portion of the washed silicasand proppant to the electromagnetic radiation further comprises:placing the at least a portion of the washed silica sand proppant on aconveyor; and operating the conveyor to pass the at least a portion ofthe washed silica sand proppant thereon through a heating chamber intowhich the local electromagnetic radiation source emits theelectromagnetic radiation.
 9. The method of claim 1, wherein theelectromagnetic radiation generated by the local electromagneticradiation source has a frequency between about 1200 MHz and about 2700MHz.
 10. The method of claim 9, wherein the at least a portion of thewashed silica sand proppant is exposed to the electromagnetic radiationfor no more than about three minutes.
 11. The method of claim 1, whereinthe silica sand proppant washed in the washer has been sorted to have apreselected particle size range prior to washing.
 12. A method ofincreasing crush resistance and fracture conductivity of silica sandproppant used in fracturing operations involving one or more hydrocarbonformations, the method comprising: washing silica sand proppant in awasher to remove impurities from the silica sand proppant; and exposingat least a portion of the washed silica sand proppant to electromagneticradiation prior to injection into a hydrocarbon well for the fracturingoperations, the electromagnetic radiation being generated by a localelectromagnetic radiation source.
 13. The method of claim 12, furthercomprising: supplying the washed and exposed silica sand proppant usedin the fracturing operations involving one or more hydrocarbonformations.
 14. The method of claim 12, further comprising: contactingthe silica sand proppant to an acidic solution in the washer.
 15. Themethod of claim 12, further comprising: contacting the silica sandproppant to an ammonium chloride solution in the washer.
 16. The methodof claim 12, further comprising: contacting the silica sand proppant toan organic removal agent in the washer.
 17. The method of claim 12,wherein exposing the at least a portion of the washed silica sandproppant to the electromagnetic radiation further comprises: placing theat least a portion of the washed silica sand proppant on a conveyor; andoperating the conveyor to pass the at least a portion of the washedsilica sand proppant thereon through a first suppression tunnel, aheating chamber and then a second suppression tunnel, theelectromagnetic radiation emitted by the local electromagnetic radiationsource into the heating chamber, the first and second suppressiontunnels positioned to reduce electromagnetic radiation leakage from theheating chamber.
 18. The method of claim 12, wherein the electromagneticradiation generated by the local electromagnetic radiation source has afrequency between about 1200 MHz and about 2700 MHz.
 19. The method ofclaim 18, wherein the at least a portion of the washed silica sandproppant is exposed to the electromagnetic radiation for no more thanabout three minutes.
 20. The method of claim 12, wherein the silica sandproppant washed in the washer has a particle size range of about 20/40mesh.